The phenomenal growth in the demand for wireless communications has put persistent pressure on cellular network operators to improve the capacity of their communication networks. To improve the spectral efficiency of these networks, scarce radio resources have to be reused aggressively in neighboring cells. One goal of the Long Term Evolution (LTE) network is to provide a network with a frequency reuse factor of one, typically denoted as N=1. A frequency reuse of 1 implies that the frequencies used by the network are available in all cells. As a result, inter-cell interference has become a main source of signal disturbance, limiting not only the service quality to users at the cell edges, but also the overall system throughput.
Coordinated Multi-Point (CoMP) transmission or reception is one technique being employed to mitigate inter-cell interference. The basic idea behind CoMP is to transmit downlink signals to a mobile terminal from multiple cells and to receive uplink signals from the mobile terminal at multiple cells. CoMP transmission and reception schemes may avoid interference by coordinating downlink transmissions or using known combining techniques to significantly improve performance, particularly for mobile terminals operating near a cell edge.
In the case of CoMP reception, the uplink transmissions from a mobile terminal are received by a serving cell and one or more cooperating cells. The set of cells including the serving cell and cooperating cells is referred to as the coordinating set. The term “cooperating cell” as used herein refers to all cells in the coordinating set. The signals received by different cells in the coordinating set may be combined using techniques such maximal ratio combining (MRC), interference rejection combining (IRC) and successive interference cancellation (SIC) to improve signal quality. The combining may be performed by the serving base station in the serving cell, or by a central processing node.
Because the cells in the coordinating set are geographically separated, the signals received by the cooperating cells need to be transported to the serving cell. If a central processing node is used, the signals received by the cooperating cells and serving cell need to be transported to the central processing node. The transmission of the received signals between nodes consumes bandwidth on the signaling network. Further, processing resources at either the serving cell or central processing node must be made available to process the received signals. However, the signals received at different cells may not be of equal value. Therefore, to avoid unnecessary waste of limited bandwidth and processing resources, it is desirable to identify and select the cells providing the highest quality signals for inclusion in the coordinating set.
In a conventional homogenous network, the six cells adjacent the serving cell are likely to provide the highest quality signals. However, heterogeneous networks deploying micro cells and relays in combination with macro cells are becoming increasingly common in order to provide better coverage in areas where signals from the macro cells may be blocked. In a heterogeneous network, there is a large variation in the coverage of different cells. Thus, it cannot be assumed that the cells closest in distance to a mobile terminal are the best candidates for cooperating cells. Further, in a mixed network, the best set of cooperating cells may change with only small movement of the mobile terminal. Accordingly, identification of the best neighbor cells to serve as cooperating cells for a mobile terminal at a given location within a serving cell is problematic.